Gas flotation is one of the most efficient and widely accepted methods used in variety of industries where removal of solid or immiscible liquid phases is of interest. In particular, in the petroleum industry, the ever-increasing volume of associated water produced from the hydrocarbon reservoirs as a side product has become a major issue to be addressed by the producers. Environmental awareness and regulations are increasingly challenging the producers to achieve a high degree of purification in the treated water streams prior to discharge or re-injection. Gas flotation has proven one of the most efficient and economical polishing processes compared to other methods and available technologies. Simultaneously, the economical penalty for additional water treatment capacity and footprint of apparatus are major factors in budgeting and decision making for the producers.
Previous endeavors include apparatus with at least two active capacities or gasification chambers with a common skimmer at the center of a horizontal tank, such as those taught in U.S. Pat. Nos. 4,990,246 and 5,348,648. Gasification is applied by means of at least two rotor assemblies located at the center of each gasification chamber. The common skimmer consists of a movable part having a downwardly opening annulus extending around a fixed part. The floating skimmer assembly must be of an exact weight, hence multiple adjustments by means of adding and removing of counterweights to the assembly is required. The assembly must be designed to specific needs of each application and must be adjusted for any variations to the froth rate, feed rate and other variables. This vulnerability to process variations is a major concern to the operation of these systems in continuous operation as constant monitoring and maintenance is required due to the unpredictable nature of production operation.
Another known flotation apparatus is disclosed in the U.S. Pat. No. 5,080,780 for a vertical single chamber unit. The system includes a vertical tank with inlet and outlet connections. The feed enters the tank from the bottom through an inlet distributor and moves upwardly and over a circular baffle arrangement to the outlet chamber. A separate gasification system is installed on the outside of the tank, including gas and liquid transfer lines, at least one pump and one vacuum pump. The pump suction is flooded by the treated liquid leaving the tank. This liquid is discharged from the pump and into the vacuum pump at specific rate and pressure dictated by the vacuum pump design. The vapor connection of the vacuum pump is piped to the top portion of the tank where the overhead vapors are drawn into the vacuum pump and mix with the motive fluid. This mixture is then released into the tank from the bottom and gas bubbles generated in this recycle line provide the flotation effect within the tank. Even though practical, this system suffers from inefficient distribution of gas, reduced capacity due to an additional recycle stream and additional space and attention required for the auxiliary gasification system.
U.S. Pat. No. 6,955,763 to Stacy, et al. describes an improved design for a gas flotation apparatus with rotors as the means for gasification. The apparatus includes two gasification chambers, each equipped with a dedicated rotor assembly, a common skimming trough in the middle of the horizontal tank and a third discharge compartment. This apparatus has a smaller footprint and lower power consumption for treatment of produced water due to a reduced number of gasification chambers and rotor assemblies from a typical four-cell unit and maintains relatively high contaminant removal efficiency. Froth can be collected in multiple locations in each chamber. The primary means of skim collection is the common reservoir integral with a partition between the two gasification chambers. Treated liquid leaves the second gasification chamber through a control valve and into the discharge chamber where additional skimming and level control mechanisms are required for final discharge of the treated liquid.
Even though these discussed systems offer improvements to the traditional mechanical or hydraulic gasification and have proven viable alternatives to such units, where a smaller footprint is desired such as on an offshore platform where space is at a premium, there still remain problems and areas of improvement driven by economic and performance criteria.
It would be desirable if an apparatus could be devised to overcome some of the problems in the conventional systems for removing suspended matter from a liquid, particularly in systems used on floating offshore hydrocarbon recovery platforms where the action of the waves upon the apparatus tends to cause the suspended matter to contaminate the recovered water. Further, it would be additionally desirable to reduce the footprint of separation systems for even stationary offshore platforms.